1. Field of the Invention
This invention relates to a a hexagonal magnetoplumbite-type barium ferrite magnetic powder and to a process for its production. More specifically, this invention relates to a magnetoplumbite-type barium ferrite magnetic powder having a specific surface area of usually 20 to 70 m.sup.2 /g and a coercive force of 200 to 2,000 Oe with a markedly enhanced saturation magnetization as compared with the prior art, which is suitable for use in high-density magnetic recording media, and to a process for its production.
2. Description of the Prior Art
In recent years, with the need for higher density magnetic recording, research and development work has been conducted on the method of perpendicular magnetic recording using magnetic recording media containing a barium ferrite magnetic powder.
The barium ferrite magnetic powder for use in the perpendicular magnetic recording method desirably has a proper coercive force (200 to 2,000 Oe) and as high a saturation magnetization as possible. It is desired moreover that the individual particles should be fine and uniform and have uniform magnetic characteristics and good dispersibility without agglomeration or sintering.
In particular, since the barium ferrite magnetic powder has lower saturation magnetization than other magnetic powders for magnetic recording media, fine barium ferrite particles having as high a saturation magnetization as possible are desired.
Various methods such as coprecipitation, glass crystallization and hydrothermal synthesis have previously been known for production of barium ferrite magnetic powders (see Japanese patent publications Nos. 50323/1985 and 15576/1985 and U.S. Pat. Nos. 4341648, 4585568, 4414124 and 4529524).
The conventional barium ferrite magnetic powders mentioned above result from substituting metallic elements such as Co, Ti, Ni, Mn, Cu, Zn, In, Ge, Nb and Zr, either alone or in combination, for part of Fe atoms of BaO.nFe.sub.2 O.sub.3 so that the total valence of the substituent atoms becomes equal to the valence of the Fe atoms substituted.
The conventional barium ferrite magnetic powders, when in the form of fine particles having a specific surface area of at least 40 m.sup.2 /g, have the defect of showing a decreased saturation magnetization of not more than 55 emu/g. This is presumably because if the specific surface area of the barium ferrite magnetic powder becomes 40 m.sup.2 /g or more, the nonmagnetic layer portion of the surface of the particles cannot be ignored. Various metal elements added for adjustment of coercivity are also considered to be one cause.
Japanese Laid-Open patent publication Nos. 236104/1986 and 16232/1987 disclose the saturation magnetization of barium ferrite magnetic powder is increased by replacing Fe atoms by Zn.
The barium ferrite magnetic powder described in Japanese Laid-Open patent publication No. 236104/1986 is obtained by the glass crystallization method. This patent document does not give any specific description on the effect of using Zn alone as the substituent atom, and completely fails to describe the crystal structure, particle size, specific surface area and coercive force of the barium ferrite magnetic powder, the control of the coercive force, and the relation of Zn to other substituent elements.
In Japanese Laid-Open patent publication No. 16232/1987, as is the case with the aforesaid conventional barium ferrite magnetic powders, it is necessary to make the total valence of substituent atoms equal to the valence of the Fe atoms substituted and thus balance the valances of the atoms. Accordingly, when the amount of Zn added is increased, the amounts of other substituent atoms for balancing the valences also increase and consequently, the saturation magnetization of the barium ferrite magnetic powder is reduced.